1. Field of the Invention
This invention relates to a sheet size detecting apparatus carried on an image forming apparatus such as a copying machine or a printer.
2. Related Background Art
For example, an image forming apparatus such as a copying machine or a printer using the electrophotographic technique forms a toner image on a recording material (sheet) such as a plain paper, and thereafter heats and fixes the toner image on the recording material by a fixing device.
Now, it is known that when small size recording materials are continuously printed at the same print intervals as large size recording materials, an area of the fixing device through which the recording materials do not pass (non-sheet passing area) excessively rises in temperature. When the non-sheet passing area of the fixing device excessively rises in temperature, parts constituting the fixing device are damaged by heat, or when a large size recording material is passed in a state in which the non-sheet passing area of the fixing device has excessively risen in temperature, there may occur a phenomenon that the toner offsets on the fixing device (high temperature offset).
So, when continuous printing is to be effected on small size recording materials, the excessive rise in the temperature of the non-sheet passing area is suppressed by taking a measure such as adopting the setting for widening the print interval more than when continuous printing is effected on large size recording materials.
To execute control for suppressing such excessive rise in the temperature of the non-sheet passing area of the fixing device, it is necessary for the image forming apparatus to recognize whether the recording material being conveyed is larger or smaller than a reference size.
FIG. 12 of the accompanying drawings shows a conventional example of means installed in an image forming apparatus for detecting the size of paper.
The letter A designates a sheet conveying path including a pair of sheet conveying rollers 4a and 4b, and in the case of this example, sheets of two kinds of sizes, i.e., a small size sheet S1 and a large size sheet S2, are conveyed by this conveying path A so that a conveyance reference O-O and the center of the sheet in the width direction thereof (direction orthogonal to a conveyance direction) may coincide with each other (center reference). A1 denotes a conveyance with area for the small size sheet S1 in the sheet conveying path A, A2 designates a conveyance width area for the large size sheet S2, and B denotes the difference area between the conveyance width areas A1 and A2 of the small size sheet S1 and the large size sheet S2.
The reference numerals 101,102 designate two sets of first and second sheet size detecting means, and the first sheet size detecting means 101 is disposed correspondingly to a location in the conveyance width area A1 for the small size sheet S1, and the second sheet size detecting means 102 is disposed correspondingly to a location in the difference area B between the conveyance width areas A1 and A2.
The first and second sheet size detecting means 101 and 102 have arms 101a and 102a, respectively, pivotally moved by the contact thereof with the sheet, and sensors 101b and 102b, respectively, for detecting the pivotal movement of the arms. In the case of the present example, the arms 1 101a and 102a are rocking members each having an upper arm portion and a lower arm portion pivotally movable about supporting shafts 101c and 102c, respectively, and the sensors 101b and 102b are photointerrupters each having a light emitting portion and a light receiving portion. The rocking members 101a and 102a are both kept in a substantially vertical upright posture by gravity in their free state.
When the sheet passed to the conveying path A is the small size sheet S1, the leading edge of the sheet S1 interferes with the upper arm portion of the rocking member 101a of the first sheet size detecting means 101. By this contact, the rocking member 101a is pivotally moved in a counter-clockwise direction about the supporting shaft 101c, and this pivotally moved state of the rocking member 101a is kept until the trailing edge of the sheet 51 has passed the position of the rocking member 101a. This pivotally moved state of the rocking member 101a is detected by the photointerrupter 101b, and the output signal of the photointerrupter changes from “open” to “close”.
On the other hand, the rocking member 102a of the second sheet size detecting means 102 is free of the contact by the conveyed sheet 51 because the location thereof is outside the conveyance width area A1 for the small size sheet 51, and the output signal of the photointerrupter 101b remain in the open signal state.
From the change of the output signal of the photointerrupter 101b of the first sheet size detecting means 101 from the open state to the closed state after the passing of the sheet has been done, and the duration of the open state of the output signal of the photointerrupter 102b of the second sheet size detecting means 102, a control circuit, not shown, judges that the passed sheet is the small size sheet S1.
When the sheet passed to the conveying path A is the large size sheet S2, the rocking members 101a and 102a of the first and second sheet size detecting means 101 and 102 are pivotally moved by the sheet S2 because the locations of both of them are within the conveyance width area A2 of the large size sheet S2, and both of the output signals of the photointerrupters 101b and 102b of the first and second sheet size detecting means 101 and 102 change form the open state to the closed state. Thereby, the control circuit, not shown, judges that the fed sheet is the large size sheet S2.
Even when the sizes of the passed sheets are three or more kinds, the number of the sheet size detecting means is increased, whereby the detection of the sizes of the sheets is possible.
However, in an image forming apparatus of a construction in which the width regulation of the sheet is effected with the center reference, and in which sheet size detecting means is provided only on one side in the width direction of the sheet, there has arisen the problem of the wrong detection of the sheet size that a sheet which should originally be detected as a small size sheet is detected as a large size sheet.
More particularly, in the case of a construction as shown in FIG. 8 of the accompanying drawings wherein for example, the width regulation of a sheet 25 is effected with the center reference, sheet passing is usually effected with the center reference with regulating guides 2 brought into contact with the sheet 25. In this case, the sensor arm 5b of a sheet width sensor is not brought down. However, when sheet passing should be done with the center reference with the regulating guides 2 brought into contact with the opposite side edges of the sheet 25, if as shown in FIG. 9 of the accompanying drawings, sheet passing is done with the regulating guides 2 not brought into contact with the opposite side edges of the sheet 25, the sheet 25 is passed while leaning toward the sensor arm 5b side of the sheet width sensor with the regulating guides 2 widely spaced apart from each other, whereby the sensor arm 5b of the sheet width sensor is brought down. As the result, there arises the problem of wrong detection that the sheet is detected as a large size sheet. This wrong detection causes such difficulties as the aforementioned “temperature rise phenomenon of the non-sheet passing portion” and “high temperature offset”. This will further to lead to the damage of the fixing device and the trouble of a main body due to the excessive rise in the temperature of the non-sheet passing portion.
To prevent such wrong detection and accurately detect the size of a sheet, arms and sensors must be provided on the left and right sides relative to the conveyance reference O-O of the sheet, and the number of the sensors has been increased to thereby increase the cost.